U.S. Pat. No. 4,850,197, which is assigned to the same assignee as the present application, discloses a vapor compression refrigeration unit based on an economizer cycle which utilizes a refrigerant compressor having an intermediate pressure port, in addition to suction and discharge ports. An economizer heat exchanger is used to enhance hot gas cooling and heating cycles which are initiated by associated electrical or electronic control to achieve and maintain a predetermined temperature range close to a selected set point temperature in a conditioned space.
U.S. Pat. No. 5,174,123, which is assigned to the same assignee as the present application, discloses refrigeration methods and apparatus which utilize a flash tank in a refrigeration unit which has an economizer cycle, in place of an economizer heat exchanger. The disclosed refrigeration arrangement eliminates the need for a float valve in the flash tank, enabling the flash tank to be used in transport refrigeration applications.
A refrigeration unit having an economizer cycle enhances the heating cycle by adding heat to the refrigerant in an economizer heat exchanger, or a flash tank, hereinafter collectively referred to simply as an economizer heat exchanger. This heat may be added to the economizer heat exchanger, for example, by circulating liquid coolant from a liquid cooled compressor prime mover, eg., an internal combustion engine, in heat exchange relation with the refrigerant in the economizer heat exchanger. During such a heating cycle hot gas refrigerant from the compressor is discharged into a condensing heat exchanger associated with the conditioned space, and the condensed refrigerant is then evaporated in the economizer heat exchanger while heat is added to the refrigerant from the liquid coolant. We have found that under certain conditions, heat is drawn from the liquid coolant in the economizer heat exchanger so rapidly that the liquid coolant can freeze.
Thus, it would be desirable, and it is an object of the invention, to improve the reliability and efficiency, as well as the control methods and arrangements, of a refrigeration unit in which a refrigerant compressor having discharge, suction and intermediate ports is driven by a liquid cooled internal combustion engine, with the refrigeration unit having an economizer cycle, such as the refrigeration units disclosed in the hereinbefore mentioned patents.
It would be desirable, and it is another object of the invention, to improve the reliability and efficiency of a refrigeration unit having an economizer cycle, during a hot gas heating cycle, notwithstanding start-up of the unit and internal combustion engine during low ambient temperature conditions, which may cause problems associated with freezing of the liquid coolant used to enhance the hot gas heating cycle.